In recent years, microwaves of 1-30 GHz and millimeter waves of 30-300 GHz have been used for information transfer, and systems utilizing high frequency signals, for instance, high-capacity communication systems at 60 GHz, or vehicle-mounted radar systems at the 76 GHz band, have been widely used. It is important, in these high frequency circuits, that are used in high frequency systems, to provide reduced-loss connections between high frequency IC's and an antenna. Particularly in systems using millimeter wave signals, the waveguide very often becomes the interface of the antenna, and broad-band high frequency line-waveguide converters with low loss are needed.
A conventional, high-frequency, line-waveguide converter typically includes a structure sandwiching a dielectric substrate, with a high frequency line, between a waveguide formed in a rectangular metallic block and a metallic short-circuit block. In the structure utilizing the short-circuit block, external leakage of electromagnetic waves in the mode conversion circuit connecting the high frequency line to the waveguide, is prevented by the short-circuit block.
In the case of installing the short-circuit block, however, there are two problems. First, the short-circuit block needs to separate parts that may cause the short-circuit. Second, the line-waveguide converter requires ample mounting space for mounting the short-circuit block.
Due to these disadvantages, a high frequency line-waveguide converter which does not use short-circuit block has been developed. However, electromagnetic waves may easily escape to the outside, and the conversion loss may be large since the short-circuit structure is constituted in a substrate having large loss and high permittivity as compared with air. Moreover, the matching range band is undesirably narrowed.